Rechargeable batteries and their use are well known in the art. Generally, a rechargeable battery is any device capable of storing a charge that is gradually depleted over time as the battery is used to drive a load. Such devices include, but are not limited to, batteries constructed from lithium material, nickel-cadmium (Ni--Cd) material or nickel metal hydride (NiMHO.sub.2) material. When the charge is at least partially (but preferably, fully) depleted, the rechargeable battery may be coupled to a charging circuit in order to recharge the battery. (As used in the context of the present invention, the terms "charging", "recharging" and their variants are used interchangeably.) A system for recharging a battery is illustrated in FIG. 1. As shown, the system comprises a charging circuit 102 electrically coupled to a battery 104 to be recharged and to an external power source 106. Operation of the charging circuit 102 usually depends on the type of battery being charged and the type of charging to be performed (i.e., rapid versus normal recharging). In general, however, the charging circuit 102 operates to provide, in a controlled manner, power from the external power source 106 to terminals of the battery 104. The flow of current at various voltage levels through the charging circuit 102 into the battery 104 causes a charge to be increasingly stored within the battery 104. Typically, the battery 104 is considered recharged when it is capable of maintaining a predetermined current level at a predetermined voltage level, i.e., constant power, without the aid of the external power source 106.
A lithium-type rechargeable battery typically exhibits favorable recharging properties in that it can be recharged through the application of a fixed voltage at the battery's terminals. A typical charging profile for lithium-type batteries is illustrated in FIG. 2. During the recharge process of lithium-type batteries, the battery voltage is allowed to increase up to a fixed threshold, and then the current from the external power supply must be gradually decreased in order to maintain this threshold. If the external power supply has a limited output voltage, then the charge current into the battery will naturally taper to a low value as illustrated in FIG. 2. The peak current during the initial charging period (when the battery is low) will be limited to the maximum output of the power source, and the power source output voltage will be pulled down to the battery voltage. The battery is fully recharged when the charge current into the battery decreases substantially to zero or a known low level.
However, in order to determine when the battery has reached such a state, the charging circuit typically comprises relatively complex circuitry for measuring the state of the battery and for controlling the voltage and currents applied to the battery's terminals. For example, conventional methods of battery metering require that the charging circuit measure voltage of the battery, and also measure and/or control current flow into the cell during the charge process. Coulomb-counting methods of battery metering require that the current flow out of the battery during discharge must also be measured. A typical prior art charging circuit 304 is illustrated in FIG. 3. A variable-output power supply 302 is electrically coupled to the charging circuit 304, which in turn is electrically coupled to a battery 306 being recharged. The variable output of the power supply 302 is determined by the battery voltage (via feedback buffer 318) during the recharge process. As the battery increases in voltage, the power supply 302 output increases proportionally in order to provide sufficient "headroom" for the desired current to flow through a sense resistor 308 and a gating device 316. Headroom addresses the fact that the additional circuitry illustrated in FIG. 3 will cause various voltage drops. In order to maintain a given voltage and current level at the battery 306, a margin accounting for these voltage drops, i.e., headroom, must be provided. Current sensing is provided by a sense resistor 308 monitored by a current sense amplifier 310. A control amplifier 312 compares the current measurement provided by the current sense amplifier 310 with a threshold 314 provided by a controller (not shown). The output of the control amplifier 312 is used to control the gating device 316 such that current applied to the battery 306 is properly controlled. Because the controller (typically, a microprocessor) actively controls the charge current through the control amplifier 312, the battery's current charge level can be determined by the amount of taper current into the battery required to maintain the constant voltage.
The additional circuitry required to measure and control current and voltage levels represent an additional cost to charging circuits. In turn, the increased cost of such charging circuits increases the cost of various devices that incorporate them. For example, wireless communications units, such as handheld radiotelephones and the like, typically incorporate such charging circuits. In this manner, a user of a wireless communication unit can simply plug their unit into an external power source and thereby recharge the batteries (either external or internal to the unit) used to power the unit. Previous realizations of portable product designs have eliminated current sensing and/or control circuitry from the device, however these types of systems have lost the capability to accurately determine the battery level, particularly during charging as demonstrated, for example, in applications of the LM3420 series of lithium-ion battery charge controller manufactured by National Semiconductor Corporation. The ability to determine battery level during recharging is a convenient feature that many users of portable devices find useful. Thus, it would be advantageous to provide a technique that solves the problem of determining a battery's level during charging without the requirement for precise current monitoring or control.